Technical Field
The present disclosure relates to a method for manufacturing a protective layer against etching with hydrofluoric acid (HF), a semiconductor device provided with the protective layer and utilization of the method for manufacturing the protective layer to make a semiconductor device.
Description of the Related Art
Surface micromachining occupies an important position in known manufacturing techniques for semiconductor devices, microelectronic devices and micro-electro-mechanical systems (MEMS). The manufacture of free-standing structures by micromachining surfaces comprises forming, on a substrate, structural layers partially overlapping sacrificial layers. Subsequent selective etching enables the sacrificial layers exposed to the etching solution to be removed to release the structural layers and form the free-standing structures.
FIGS. 1-6 show the manufacturing steps of an inertial sensor 1, in particular a gyroscope, according to a known process. In particular, a process is shown for forming the free-standing structures of the stator and rotor, in epitaxial polysilicon (also known as “EPIPoly”), above a silicon substrate that houses conductive strips in polysilicon able to form electrical connections to and from the free-standing structures.
With reference to FIG. 1, according to a method of manufacture of known type for making a gyroscope, a silicon substrate 1 is provided. A silicon oxide support layer 2 is then formed, through thermal growth for example. This support layer 2 is also known as the permanent or field oxide layer and has an approximate thickness of between 2 and 3 μm. The support layer 2 has the function of supporting the overlying structures (formed in a subsequent step) and is capable of reducing parasitic capacitance between these overlying structures and the underlying substrate 1.
A layer of doped polysilicon (N-type for example) is formed over the support layer 2, which is subsequently etched to remove selective portions of the polysilicon and form electrical contact regions 4a, 4b. The electrical contact regions 4a, 4b are conductive strips and form, as will be further explained in the subsequent manufacturing steps, electrical interconnections. The etching of the polysilicon layer, to form the electrical contact regions 4a, 4b, is of the selective type and does not remove portions of the support layer 2. As previously mentioned, the support layer 2 has the function of electrically insulating the electrical contact regions 4a, 4b from the substrate 1 and reducing parasitic capacitances on the latter.
Then, FIG. 2, a sacrificial layer 6 of silicon oxide is formed (for example, by PECVD) over the support layer 2 and the electrical contact regions 4a, 4b. Portions of the sacrificial layer 6 from over the underlying electrical contact regions 4a, 4b are removed through lithographic steps and subsequent etching, forming pluralities of trenches 8 that extend towards the electrical contact regions 4a, 4b, so as to expose respective surface portions of the electrical contact regions 4a, 4b. In particular, two trenches 8 are formed over the electrical contact region 4b. 
During the step in FIG. 2, a trench 9 is also formed that extends though the sacrificial layer 6 and the support layer 2, until the upper surface of the substrate 1 is reached and exposed. In subsequent manufacturing steps, this trench provides the passage for forming a ground terminal in electrical contact with the substrate 1.
Then, FIG. 3, a structural layer 10, for example of epitaxial polysilicon (“EPIPoly”), is formed over the sacrificial layer 6 and in the trenches 8, 9, extending in trenches 8 to make electrical contact with the electrical contact regions 4a, 4b, and in trench 9 to make electrical contact with the substrate 1. The structural layer 10 can be processed as needed, to form structures having a desired conformation.
In FIG. 4, the structural layer 10 is selectively etched to form free-standing structures movable in one or more directions (a stator 11 and a rotor 12), lateral walls 13 able to delimit a chamber 14 that houses the stator 11 and the rotor 12, and electrical contact terminals 15 outside of the chamber 14 (only one electrical contact terminal 15 (“pad”) is shown in FIG. 4).
However, it should be noted that in this manufacturing step, the stator 11 and rotor 12 are still constrained to the underlying sacrificial layer 6 and therefore are not free to move. Through holes 18 are also formed in the structure of the stator 11 and the rotor 12 to enable the removal, in subsequent manufacturing steps, of the sacrificial layer 6, so as to partially suspend the stator 11 and the rotor 12. This process step is shown in FIG. 5, where the stator 11 and rotor 12 are rendered free-standing by removing portions of the sacrificial layer 6 that extend beneath them. The portions of the stator 11 and the rotor 12 that, in FIG. 4, extend into the trenches 8 form, in FIG. 5, support bases 16, 17 for the stator 11 and the rotor 12, respectively. These support bases 16, 17 are also in electrical contact with the underlying electrical contact regions 4a, 4b. 
As can be noted in FIG. 5, part of the sacrificial layer 6 remains beneath portions of the lateral walls 13, to support them and provide adequate electrical insulation of the lateral walls from the electrical contact region 4b. 
In addition, to protect portions of the electrical contact region 4b that remain exposed to the outside environment at the end of the manufacturing steps, a silicon nitride (SiN) depositing step is performed to cover and protect the electrical contact region 4b (the protective layer 3 can be seen in FIG. 5).
Finally, FIG. 6, the manufacture of the inertial sensor (here, for example, a gyroscope) is completed by placing a cap 19 on, and in contact with, the lateral walls 13. The cap 19 and the lateral walls 13 are coupled to each other by solder material 20, of the conductive or insulating type according to preferences. In this way, the chamber 14 is insulated to protect the stator 11 and the rotor 12 and, in general, all of the elements (movable and fixed parts) that form the gyroscope and are not shown here in detail. As mentioned, there are electrical contact terminals 15 outside the chamber 14 that are electrically connected to respective electrical contact regions 4a, 4b to receive/feed electrical signals from/to the stator 11 and rotor 12.
The etching step to remove the portions of the sacrificial layer 6 (of silicon oxide) that extend beneath the stator 11 and rotor 12 is typically etching using hydrofluoric acid (HF) in the vapor phase or, alternatively, wet etching using an HF solution or mixture. The hydrofluoric acid etches the silicon oxide in an isotropic manner, but not the polysilicon. Therefore, stator 11 and rotor 12 are not damaged. The etching of the sacrificial layer 6 with HF can be halted in a region close to the interface between the sacrificial layer 6 and the support layer 2 by knowing the etching rate and monitoring the etching time; alternatively, the another solution is to use an etch stop layer, arranged between the sacrificial layer 6 and the support layer 2, chosen in a material that is etch-resistant to HF and which does not allow HF to penetrate through it.
However, the first solution (monitoring the etching time) is not optimal and, in general, is not applicable, as complete and uniform removal of the sacrificial layer 6 cannot be ensured in all situations.
The second solution is not, in actual fact, practicable or practiced, as known HF-resistant materials exhibit a series of other contraindications.
For example, silicon carbide (SiC), silicon-germanium (SiGe) and polysilicon-germanium (Poly SiGe) are materials that can be used as an etch stop layer, because they are resistant to hydrofluoric acid.
Other materials, such as silicon nitride (SiN), are not resistant to hydrofluoric acid. In particular, when using HF vapor etching, in addition to being removed, SiN forms salts with the hydrofluoric acid vapors that cause high defect rates in the final structure. The use of SiC, for example deposited using PECVD (Plasma-Enhanced Chemical Vapor Deposition), although giving resistance against HF etching in certain conditions, does not provide suitable impermeability to HF because, if deposited on defined structures, it can give rise to micro-cracks. Infiltrations of hydrofluoric acid can thus occur through the SiC layer, which cause etching of the underlying support layer 2. SiC also has other undesired characteristics that appear, in particular, after any annealing that may be performed by the manufacturing steps following deposition of the SiC layer. In particular, reduced adhesion of SiC to silicon oxide and a change in the insulating properties of SiC, which acquires a conductive behavior, have been observed after annealing.
SiGe, although both resistant to HF etching and impermeable to HF, typically requires a high level of purity (absence of doping impurities). Vice versa, the diffusion of any doping species drastically reduces the dielectric constant value of SiGe, making it unsuited to applications (as in the case shown on FIGS. 1-6) in which high electrical insulation is desired between the electrical contact regions 4a, 4b and the underlying layers. A similar discourse holds for polysilicon-germanium.
In consequence, to ensure complete etching of the sacrificial layer 6 without jeopardizing the electrical and structural characteristics of the other layers, it is normally preferred to completely etch the sacrificial layer 6 and partially etch the support layer 2. As HF etching is of the isotropic type, a phenomenon of etching beneath electrical contact regions 4a, 4b (known as underetch or undercut) is observed, which creates free-standing peripheral portions of the electrical contact regions 4a, 4b (generically indicated in FIG. 6 by reference numeral 4′). This fact can cause problems of weakening or possible breakage of the free-standing portions 4′. This results in an intrinsic design limit on the size of the electrical contact regions 4a, 4b that, in order to contain underetch and therefore the portion of free-standing region, does not allow the size of the electrical contact regions 4a, 4b to be reduced (or rather, there is a limit on rescaling the device). In fact, for very narrow electrical contact regions 4a, 4b, underetch could cause irreparable damage to the support layer 2 beneath them. Furthermore, there are also layout complications that take the size and extent of underetch into account to avoid them becoming excessive.
As well as being a limit to reducing the size of the device, the free-standing regions 4′ are mechanically fragile and can break in cases where there is contact with the overlying moving structures, situations that typically occur if the devices are in free fall (which can happen, depending on their application) or severe impacts.
The problems described herein can also be encountered in the case of generic inertial sensors, different from the gyroscope, such as accelerometers for example, or devices equipped with a free-standing mass in general.